1. Field of the Invention
The present invention relates to a laminated piezoelectric/electrostrictive device which is excellent in durability and which is drivable over a long period.
2. Description of the Related Art
To solve a global warming problem as one of global environmental problems, a technology to reduce fuel consumption of an automobile has been advanced. As an eco-friendly power source of the automobile, a fuel cell is regarded as effective, but there are many technical problems and cost problems in realizing such a fuel cell. To solve the problem, a hybrid engine system in which an electric motor is used together has been increasingly put to practical use. Especially in Europe, an automobile on which an improved diesel engine having emissions of harmful substances suppressed is mounted largely increases its share.
The diesel engine is an engine in which a fuel is jetted into a cylinder and compressed at a high pressure to self-ignite, and an engine output is controlled in accordance with an amount of the fuel to be jetted. Therefore, in the diesel engine, a function of a fuel jet device which feeds the fuel into the cylinder is important. In recent years, a reason why the use of the diesel engine increases largely lies in development of a so-called common rail type fuel jet device. Especially in the future, it is expected that enlargement of the use of the diesel engine is supported by employing a piezoelectric device as the driving source of a mechanism for opening or closing a jet nozzle, instead of a solenoid. This is because the piezoelectric device can realize the opening/closing of the jet nozzle at a higher speed and with a higher precision as compared with the solenoid, and a more appropriate amount of the fuel can be fed into the cylinder at a more appropriate timing to form an optimum combustion state of the fuel. As a result, it is considered that excellent effects are brought about such as reduction of harmful substances such as particulates, nitrogen oxide, carbon monoxide and hydrocarbons included in an exhaust gas, the reduction of the fuel consumption, turning-down of an engine sound and enhancement of the engine output.
In many cases, as the piezoelectric device to be applied, to the fuel jet device, as the driving source (micro actuator to open or close the nozzle) of the opening/closing mechanism of the jet nozzle, a laminated type of piezoelectric device is employed because a displacement amount can be enlarged. This laminated type of piezoelectric device in general has a structure in which a pair of outer electrodes connecting inner electrodes having the same polarity to each other are formed on a peripheral surface of a laminated article constituted by laminating a plurality of layered piezoelectric articles and inner electrodes.
To prevent a shortcircuit from being caused, such a laminated type of piezoelectric device takes any of the following configurations in many cases. In a first configuration, in principle, the piezoelectric articles and the inner electrodes having an equal size are laminated, but in a place in the vicinity of the peripheral surface where one inner electrode is connected to one outer electrode, another inner electrode having a different polarity is laminated so as to be retracted in the laminated article so that the other inner electrode does not appear on the peripheral surface of the laminated article. Similarly, in a place in the vicinity of the peripheral surface where the other inner electrode is connected to another outer electrode, the one inner electrode having the polarity different from that of the other inner electrode is laminated so as to be retracted in the laminated article so that the one inner electrode does not appear on the peripheral surface of the laminated article. In a second configuration, in principle, all the inner electrodes which are smaller than the piezoelectric article are laminated so as to be retracted in the laminated article so that the inner electrodes do not appear on the peripheral surface of the laminated article. In a place where one inner electrode is connected to one outer electrode, however, the one inner electrode appears on the peripheral surface of the laminated article and is connected to the one outer electrode. In another place, another inner electrode is similarly connected to another outer electrode. It is to be noted that examples of prior documents on the laminated type of piezoelectric device include Patent Documents 1 to 3.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2005-223013
[Patent Document 2] Japanese Patent Application Laid-Open No. 2001-102647
[Patent Document 3] Japanese Patent Application Laid-Open No. 6-164009
[Patent Document 4] Japanese Patent Application Laid-Open No. 2002-160195
In any of the above-described configurations of the laminated type of piezoelectric device, in the place where one inner electrode is connected to the outer electrode, any inner electrode is not disposed in one of a lower layer and an upper layer of the portion between which the piezoelectric article is sandwiched. Therefore, the portion is an inactive portion in which any displacement is not generated in the piezoelectric article. Therefore, in response to the displacement generated in an active portion of the piezoelectric article sandwiched between the inner electrodes, a large stress is generated in the inactive portion, or the stress is concentrated on a boundary between the active portion and the inactive portion. Therefore, when the laminated type of piezoelectric device is used over a long period, cracks are generated in the piezoelectric article owing to the stress concentration to deteriorate a performance, and a shortcircuit is caused between the inner electrodes owing to the generation of the cracks. The device cannot be driven in some case.
Especially, when a large displacement amount is required as in the piezoelectric device to be applied as the driving source of the opening/closing mechanism of the jet nozzle in the fuel jet device of the diesel engine, in order to obtain a desired displacement amount, the number of the laminated piezoelectric articles and the number of the laminated inner electrodes are large in the laminated type of piezoelectric device. In such a laminated type of piezoelectric device, the stress necessarily enlarges, and many inactive portions are present. Therefore, it has been difficult to realize the laminated type of piezoelectric device having both a high durability and a high performance.